Electric sound reproducer



1927. May 31 A. At THOMAS ELECTRIC SOUND REPRODUCER Filed March 10. 1926 "I f 2924) /4 /5 28 F 35 f fg! Patented May 31, 1927.

,PATENT ol?" ADOLIH A. THOMAS, OF NEW YORK, N. Y.

FICE.

ELECTRIC SOUND REPRODUCER.

Application filed March 10, 1926. Serial No. 93,547.

My invention is for a new and improved electric soundn reproducer of simple yconstruction and possessing a high degree of sensitivity. The instrument of my invention may be used for recording, transmitting and reproducing sound-wave energy, but for brevity I shall refer to it simply as a sound reproducer without thereby intending any restriction. At the present time my invention has special advantages as an electric phonograph reproducer, particularly for use on records having a microscopic groove of several hundred threads to the inch, Where the vibrations of the needle point `are too delicate for satisfactory mechanical re reduction.

It has een proposed to make phonograph records with a spiral ktrack so fine and operating at such low speed that a twelve-inch record will play ,for an hour and even'longer. Mechanical reproduction from vsuch records is obviously out of the question. A record groove 'having several hundred threads Ato the inch is too,delicate for a needle point connected to a stiff diaphragm, as in the old phonograph sound boxes familiar to all. Electric reproducers of' the microphone type (also using a diaphragm) nave een used on phonograph records of :he old style-that is, records'having about L lines to the inch, where the groove is :omparatively deep and wide. While a groove like that is capable of aptuating a needle against the high tension and resist- `nce of a stiff diaphragm, a very differentN roblem arises in the case of a record having rom to 500 threadsV to the inch and of orre'sponding lineness in width and depth. i: groove ofsuch microscopic dimensions 'ill not stand the wear and tear inevitably ttending the use of a reproducer in which 1e needle operates a'diaphragm, whether ie latter loe purely mechanical or part of an ectrical reproducing mechanism.

It is the purpose of this invention to prode an electric reproducer especially adaptl for phonographic use, in which the vibraons oi the needle point are transmitted in 'eatly amplified ratio to a. ivoted coil or te member arranged to osci late in a' magitic field. In the preferred form of the vention, the inner end `oi the needle arm connected to the pivot shaft of the coil by thread or liber, which eliminates all lost ation. Suitably arranged resilient means normally hold the coil and needle arm in predetermined position and always maintain the connecting thread taut. hese resilient means, the tension of which is adjusted to the right degree, offer practically the only resistance to a needle point in the record groove, and this resistance may be very small, depending upon the physical characteristics of the groove, The coil is not attached to, or connected with, a movable iron core or armature. In other words, the coil is coreless and therefore it can be made so light as to have no appreciable inertia in its oscillations. Nor is' there any magnetic drag or load imposed upon the needle arm b v the electromagnetic reproducing mechanism. Hence, the needle arm is free to respond correctly to the minutest vibrations of the needle point, which may be as small as (and even smaller than) a thousandth of an inch. The mechanical amplification between the needle point and the sides of the pivoted coil is such that the smallest needle vibrations oscillate the coil to cut the magnetic flux and thereby generate current impulses,

which are electrically amplified to operate,

a loudspeakr or other suitable device.

In order to explain my invention fully skilled in the art practise the same, I e accompanying drawings in which I have illustrated a preferred embodiment of the invention when used as'a phonograph reproducer. In these drawings,

Fig. 1 shows an inside face view of an electric phonograph reproducer constructed in accordance with my invention, certain parts being for clearness cut away on the Vbroken line 1-1 of Fig. 2;v

Fig. 2 represents a sectional view taken approximately on line 2 2 of Fig. 1;

Fig. 3 is a transversecross-section on the broken line 3-3 of Fig. 1; f

and clearly, so that those may understand and shall refer to th Fig. 4 is a similar cross-section on line 4-4 of Fig. 1

Fig: 5 illustrates the coil-supporting frame in detached erspective view; and

Fig. 6 indicates diagrammatically the amplifying transmission between the needle arm and the pivoted coil.

.Before proceeding with a detailed description of these figures, I want to explain that the various parts have been purposely drawn in ossly exaggerated proportions, so as to magi; the drawings-clear. and easy to read.

The actual instrument is made very small and compact; indeed, it need be no bigger thanV an ordinary watch.

The different parts of the mechanismare mounted in a casing indicated as a whole by C and constructed of suitable non-magnetic material, such as aluminum, brass and the like. In the present instance, the casing consists of a back plate 1, a cylindrical bo y section 2, and a. front plate or cover 3. These parts are separately secured together by screws 4 or otherwise.

If the case' is made of moldable material, like bakelite for instance, the parts 1 and 2 may be cast as a single cup-shaped member. The back plate 1 has a hub 5 adapted to fit tightly over a tubular supporting arm 6, which is supposed to beso mounted that the reproducer travels over a record in playing position. Suitable means, such as a setscrew 7 on arm 6 and a bayonet slot 8 on hub 5, holds the casing in proper position on the supporting arm.

In casing C is mounted a magnet M, here shown as a permanent steel magnet of circular form to t closely around the periphery of the casing. This permits the use of a magnet of maximum length in a small space. Manifestly, instead of a permanent magnet, any other ractical means may be employed to establish a normally constant magnetic field. In the present construction, it is convenient to attach the magnet to the back plate 1, as by screws 9 orotherwise.

' The polar ends of the magnet are bent forwardly, as shown at 10, and to these polar ends are rigidly secured pole pieces 11 by screws 12. The pole pieces 11 are usually made of soft iron or other material of high magnetic permeability, and their opposing ends terminate in extensions 13, as es shown in Fig. 3.

In the airgap 14 formed between the polar extension 13 oseillates a small. coil 15 mounted on a pivoted shaft 16. There is no iron core or other heavy mass movable with the coil, so that the inertia of this oscillatory unit can be made so low as to be practically negligible. The arc-shaped polar extensions 13 provide a substantially uniform field across the airgap, as will be understood from Fig. 3, and the width of this uniform eld is suiicient to include the maximum oscillations of coil 15. This feature of a coil free from a movable iron mass and mounted to oscillate without appreciable inertia in a practically uniform magnetic field is 0f considerable importance,for it adds to the sensitiveness and accuracy of the instrument. In the preferred form of my invention. the coil is supported on the magnet itself, so that the coil and magnet can be properly assembled and adjusted before insertion into the casing. For this purpose, I provide a non-magnetic frame indicated Aas a .whole by to the needle arm.

'33 mounted in the F. This frame is rectangular in form, and the side bars 17 thereof are recessed or offsetat 18 to receive the pole pieces 11, to which the entire frame (with all the parts it carries) is secured by screws 19. The coil shaft 16 is journalled in adjustable bearings 20 in the upper and lower cross-bars 21 yof frame F. These bearings are preferably jewel bearings or of equally practical construction. The offset recesses 18 in frame F bring coil 15 into central alignment in the airgap 14, as may be seen from Fig. 3. A light coil spring 22 is secured at'one end to the coil shaft 16 and at the other end to a pin' or lug 23 on frame F.y The purpose of sprin 22 is not only to hold coil 15 in normar? predetermined position, but also to positively actuate the coil in one direction, as will presently by explained. The normal tension of spring 22 may be regulated by any suitable means, as by passing its outer end adjustably through a slot in lug the adjusted end in position.

j A needle arm 24 is pivoted to casing C in any practical way. In the drawings I have shown a pair of lugs 25 projecting from the lower end of the casing, and in these lugs are mounted pivot bearings 26 between which the needle arm is supporte for vibratory movement. The lower end of needle arm 24 is in the form of a needle holder 27. A slot 28 in the body section 2 of the casing permits the needle arm to extend freely into the casing. The inner end of needle arm 24 is connected to the coil shaft 16 in any practical way. At the present time I prefer a lilar connection 29 in the form of a thread, fiber, line wire, and the like. For the sake of brevity, I

t shall refer to the connection 29 as a thread,

it being understood that I am using this term in its broadest possible sense. he thread 29 is fastened at one end to the needle arm 24, as indicated at 30, and the other end of the thread is attached to shaft 16 of the coil. This is perhaps most conveniently done by providing a very small pulley 31 on shaft 16. The thread is fastened to the pulley, wound around it a few times, and the other end is then connected There is thus no lost motion between coil 15 and needle arm 24, and thread 29 is not affected by temperature conditions.

A resilient wire 32 (a thin piano wire will do) is stretched transversely across the casing and is connected to the needle arm-as by passing in a tight fit through a small hole in the arm. The ends of the tensioned wire 32 are secured to adjustable anchor pieces body section 2 of the casing. The normal tension of spring 22 and wire 32 is such that normally the coil 15 and needle arm 24 are held in predeter- 23 and securing das '3.. That 1s to say, .spring 22 and wire 32 is to oppose each the needle arm away from the magnet the `thread 29 mined position, as illustrated in Figs. 2 and the normal tendency of other, the wire 82 normally tending to ull an thus to rotate the coil clockwise, vas viewed in Fig. 3), and the coil sprin 22 normally tending to turn the coil in t e reverse di# rect-ion. Consequently, the thread 29 is always under tension, possible between the'needle arm 24 and the oscillating coil 15. This is important, because the` vibrations of the needle arm, even at maximum amplitude, are very small, and

t they'must be transmitted without loss to the coil. Y

The operation of the mechanism above described will now be understood by those skilled in the art, but I may briefly summarize it in a few sentences; As the needle arm vibrates during the playing of a record, the inner end of the arm moves'toward and away fromthe magnet. Whenever the arm moves away from the magnet, it pulls and this turns the coil (clockwise as viewed in Fig. 3) out of its ,l normal position, thereby causing it to vcut v however, the

Ato .the

1 Aadd to the inertia of the i or describe any the magnetic lines of force and generate a current impulse. This movement of the needle' arm and coil is against the tension et spring 22, which is thus slightly wound up. Now, as the inner end of the needle arm moves toward the magnet, the tension on thread 29 is relaxed and the tensioned coil spring' 22 immediately turns the coil in .the o posite direction as far as the position o the needle arm permits, thereby generating another current impulse. It will thus be seen that the needle arm positively actuates the oscillating coil 15 in one direction, and the spring 22 positively actuates the coil in they opposite direction. 'lhese oscillations of thecoil generate current impulses which are electrical re resentations of the needle vibrations. ttention is called to the fact that the oscillations of the coil are not dampened or impeded by mechanical inertia or magnetic drag, since there is no magnetic mass movable with the coil. l might explain that, when Irefer coil 15 as coreless, I mean that it 1s not mounted on, or connected to, or otherwise provided with, an iron core or armature or other heavy mass that moves with the coil. I do notv mean vto exclude, use of a stationary piece of iron between the polepieces and within the space defined by the movements of the coil, since Vsuch a `fixed magnetic piece would not coil and would be employed merely to concentrate the magnetic lines.

I have not deemed it necessary to show circuit connections for coil 15, because such connections form no part vice.

and no lost motion isy of th without intending it as a restriction.

of my invention, understood by those familiar with this art how to utilize electric current impulses to operate a loudspeaker or other acoustic dehave shown a pair of insulated terminals or binding posts 34 mounted on the back plate 1 of the casing and to which the ends of coil 15 are connected. Conductors 35 lead from the binding posts 34 through the hollow arm 6 to a suitable amplifying system for properly amplifying the feeble current impulses generated by the oscillating coil 15. arious amplifying circuits are well known in the art.

Attention is directed to the ktact that the minute vibrations of the needle point are mechanically transmitted to the coil in greatly amplified ratio: F irst,` there is the amplified needle arm in relation the needle point; This be from 3:1 to 5 mg upon the relative lengths of the needle arm on opposite sides-of the pivot.v Then, there is the multiplying connection between the needle arm and the coil. A very short pull on the thread 29lin one direction, ora very small vturn of the coil shait 16 by the spring 22 in the other direction, swings the two sides of the coil through an are that cuts the magnetic ldux even when the angular movement to the movements of ampliing ratio may :1, depen be understood from the kdiagram in lig. 6. Let us imagine that the line 36 represents the normal position of coil 15. and that the point 37 represents the axis of the coil. N ow, let us suppose that the' needle arm pulls the thread an amount indicated bythe line 88, so that the coil turnsv into the position shown by the heavy line 39. In other words, the coil has moved through an angle 40, which is relatively very small, but the opposite sides a and 5 et thecoil have opened to the magnetic dus a wide arearepresentedfby (or proportional to) the line l1, which is the elective coil area at that moment. Roughly speaking. therefore, line 38 represents between the needle arm 24 and the coil 15. In the present instance, this multiplying connection is aboutf1:20, as measured 1n Fig. 6, and that maybe taken as a fair illustrat'ilplp, e shortest possible pull on thread 29, as when the needle point vibrates with an amplitude of less than a thousandth of an inch, is sulficient to move the coil 15 to cut magnetic lines of force. It goes without saying that the lines in Fig. 6 have been purposely exaggerated in relative proportionsto-make the iigure clear. In the actual instrument. the angular movements small. indeed.

point and the inner It is Suiiicient for this case that I movement of the inner end of the of the coil are very and also because it is well i e coil is a minimum. This will easily the ratio of line 41 to the multiplying connection v Vand wire 32, which always each other, offer just enough end of the 4needle arm to be about 1:3 (a` very practical ratio), and further assuming the multiplying connection between the needle arm and the sides of the coil to be about'l 20, it is clear that the needle vibrations are transmitted to the coil in an amplified ratio of about l 60.

It is advantageous to hold the coil 15 normally in neutral or zero position, as shown in Fig. 3, where practically no lines of force pass through it, because under those conditions the oscillatory movements.of the coil give maximum response. That is so for the reason that the value of an induced electromotive force isA proportional to the rate at which magnetic lines of force are cut by the moving conductor, and the lines of force are cut most rapidly when the curve of magnetic flux passes through zero.

It should be observed that the coil land magnet M exert no magnetic drag on the needle arm, thus leaving the latter free to be vibratedin strict accordance with the undulatory track on the record. The spring 22 tend to balance resistance to the lmovement of the needle pointto impose the proper load on the needle arm. Since the coil 15 is made very small and exceedingly light, its inertia may be considered negligible. This increases the sensitivity and precision of the instrument.

In playing a phonograph record having several hundred lines to the inch, the pressure of the needle point in the record groove should be much less than in the case of the old-type records having comparatively deep and thick-walled grooves. If the normal wei ht of the reproducer on account of magnet (the only heavy part it'contains) is greater than it should be to obtain the best results from a finely grooved record, the effective pressure of the needle point can be regulated as desired by a 'counteracting weight'` on arm 6. or by any other practical means. The exact pressure with which the needle point should track the record groove for the most satisfactory reproduction is a matter of trial and experiment.

In the broader aspect of my invention, the vibratory coil l5 may be connected to an acoustic diaphragm set in motion' by sound waves, so that the instrumentJ will operate as a telephone transmitter, particularly7 useful for broadcasting. The operation of coil 15 with a diaphragm is the same asv above described. Also, it will be clear that when current impulses are passed through coil l5, the same willpos'cillate' in the magnetic airgap and cause vibrationof the needle arm 24 or of a diaphragm connected to the coil. In that -event, the instrument can be used as a -phonographic recorder or as a telephone receiver. All these and other uses are within thescope of my invention. The particular instrument herein shown and described is merely an illustrative embodiment .of my invention, and is not to be re arded as a restriction or limitation thereo What I claim as my invention is:

1. In an electric phonograph reproducer, means for establishing a magnetic field. a coreless coil mounted centrally on a rotary Shaft to oscillate in said field, a needle arm pivoted near one end so that the needle vibrations are transmitted to the other end in amplified ratio, and an amplifying connection between said other end of the arm and said shaft.

2. An electric phonograph .reproducer comprising a casing, a magnet structure mounted in said casing and having an airgap, a non-magnetic frame secured to said structure, a coil pivotally mounted on said frame so as to oscillate in said airgap, and a needle arm carried by said casing for oscillating said coil.

V3. An electric phonograph reproducer having a needle arm, a coil mounted on a rotary shaft so as to oscillate in a magnetic field, and connectingmeans between said arm and said shaft for transmittin the movements ofthe arm to the sides o said coil in amplified ratio.

4. An electric phonograph reproducer having Aa centrally pivoted coil arranged to oscillate in a magnetic field, and connections whereby the needle vibrations are mechanically transmitted to the flux-cutting sides of said coil in amplified' ratio, said connections including a taut lar member adapted to prevent lost motion.

5. In an electricV acoustic device, a centrally pivoted coil mounted to oscillate in a magnetic field, an acoustically operated member, a flexible connection between said member and said coil for positively actuating the coil in one direction only. and means independent of said member for positively actuating said coil in the other direction, said means being controlled by the movements of said member.

6. An electric sound transmitter including a vibratory member, a coil mounted toA oscillate about a central shaft in a magnetic field to cut the lines 0f force, a filar connection between said member and said shaft to swing said coil in one direction, and a spring for holding said lar connection under tension and swinging said coil in the opposite direction.

7. An' electric phonograph reproducer having a coil mounted centrally on a rotary shaft so as to oscillate in a magnetic-field, a needle arm, a filar connection between said arm and said shaft for positively rotating the latter in one direction, a s ring connected to said shaft for rotating t esame in the reverse direction, and resilient means acting `in opposition to said spring for holding the coil and needle arm in normal predetermined position.

8. In an electric phonograph reproducer, a. vibratory needle arm, a pivoted electromagnetic member mounted independently of said arm, a flexible connection between said arm and said member for causing the arm to actuate said member positively in one direction, spring means for positively actuating said member in the opposite direction, and resilent means connected to said arm and adapted to cooperate with said spring means for holding said arm and said member in normal predetermined position.

9. An electric acoustic instrument comprising a pivoted shaft, a coil fixed centrally on said shaft, means for establishing a magnetic field in which said coil is mounted to cut the lines of force, an acoustically actuated member, a filar connection wound around said shaft and attached to said member, so that movement of said member away from said shaft produces rotation of the shaft and chil in one direction, a spring connected to said shaft for rotating the same in the opposite direction when said member moves toward the shaft, whereby the vibrations of said member produce oscillation of said coil with practically no lost motion and with amplified effect tow generate current impulses in the circuit of said coil, said cil being also adapted tooscillate when current impulses are sent therethrough to actuate said member. f

10. In an electric a needle arm, including a phonograph reproducer,

centrally pivoted element f mounted independently of said arm and operatively connected thereto, and a pair of oppositely acting resilient members for holding said arm and said element in normal predetermined position, one of said members being connected to said arm and the other member being connected to said element.

11. A n electric sound transmitted comprising a magnet havin an airgap, a nonmagnetic frame secur to said magnet, a coil pivoted on said frame for oscillatory movement in the magnetic airgap, and a vibratory acoustic member connected to saidcoil in ampliedtransmission ratio.

12. An electric phonograph reproducer comprising a permanent magnet, pole pieces secured to said maghetto provide an air ap, a non-magnetic frame mounted on sai pole pieces, a coil pivoted on said frame for oscillatory movement in the magnetic airgap, and a needlearm for actuating said coil. f

13. In an electric phonograph reproducer having a magnet, a unitary structure adapted to be mounted as a single member on said electromagnetic mechanism Y magnet, said unitary structure comprising a pair of pole pieces adapted to be directly attached to the polarl ends of said magnet, a non-magnetic frame rigidly connected to said pole pieces, and a coil centrally pivoted on said frame so pole pieces. I

14. In an electric phonograph reproducer, a casing in which is mounted a pivotedv coil for oscillation in a magnetic airga a needle arm connected to said coil by a exible member, a spring connected to said coil, and a tensioned wire stretched transversely across said casing and at the center engaging said arm, said spring and wire acting in opposite directions to normally hold said coil and said arm in predetermined position.

15. An electric phonograph reproducer comprising a magnet having an airgap, a non-magnetic frame, a shaft journalled in said frame, a coil fixed to said shaft so as to oscillate in said airgap,

connected at one end to said frame and at the as to oscillate between said a helical spring other end to said shaft for normally holding the coil in predetermined position in the magnetic' airgap, a needle arm, a flexible connection between said shaft and the inner end of said needle arm, and a resilient tension member connected to said needle arm for holding mined position.

16. An electric acoustic device having a coil mounted centrally on a rotary shaft so as to oscillate in a magnetic field, a vibratory acoustic member, a thread or fine wire for operatively connecting said vshaft and said member, and resilient means for normally holding said coil in predetermined positio 17. In an electric phonograph reproducer, a casing adapted to be mounted on a suitable support, a magnet secured to a wall of said casing in such position that the magnetic airgap is near one end of the casing, a needle arm pivoted near one end to said casing at a point diametrically opposite the same in normal predetersaid airgap, so that the needle vibrations i plifying connection between said coil and the inner end of said needle arm.

18. An electric acoustic device having a coil mounted to oscillate on a rotary shaft 1n a magnetic field, a vibratory acoustic member, a ilar connection between said member and the shaft of said coil for oscillating the coil in one direction, and means for osclllating the coll 1n the reverse direction.

ADOLPH A. THOMAS. 

